1. Field of the Invention
The present invention relates to a band elimination filter used in a microwave band, in particular, to a band elimination filter composed of a microstrip line.
2. Description of the Related Art
An example of a conventional microstrip band elimination filter is shown in FIG. 4. Referring to FIG. 4, 1/4.lambda. stubs are vertically disposed on a microstrip main line at intervals of 1/4.lambda.. In other words, in FIG. 4, three 1/4.lambda. open stubs 11, 12, and 13 are connected to a microstrip main line 10 at intervals of 1/4.lambda. so as to form a three-staged microstrip band elimination filter. This microstrip band elimination filter is disclosed in for example Japanese Patent Laid-open Publication No. 57-10507. In addition, another microstrip band elimination filter is disclosed in Japanese Patent Laid-open Publication No. 63-212201 as a second related art reference of the present invention. In the second related art reference, a 1/4.lambda. resonator is field-coupled with a microstrip main line so as to form a band elimination filter.
As shown in FIG. 4, in the conventional microstrip band elimination filter, 1/4.lambda. stabs are vertically disposed on a microstrip main line. The 1/4.lambda. stubs and the microstrip main line are formed on a dielectric substrate. In this structure, the area of the portion other than the pattern of the band elimination filter is large. Thus, although the drawback of the structure of the three-staged band elimination filter shown in FIG. 4 is not remarkable, as the number of stages increases, the area of the band elimination filter becomes large.
Consequently, the size and weight of the band elimination filter cannot be reduced.